1. Technical Field
The present invention relates to a laser light source including a wavelength converting element for performing wavelength conversion utilizing a non-linear optical effect, and a method for controlling the laser light source, and also relates to an image display apparatus and a processing apparatus incorporated with the laser light source.
2. Background Art
Laser light is used in various fields such as measuring apparatuses, medical apparatuses, processing apparatuses, and display apparatuses, not to mention communications apparatus and optical recording apparatuses. A semiconductor laser is used as a primary light source in these fields in view of control feasibility and its small size.
Among the above light sources, a semiconductor laser for stably generating light of a wavelength in a visible region from 0.5 μm to 0.6 μm has not been realized. A semiconductor laser for generating light of a wavelength in a mid infrared region of 2 μm or more is required to be cooled in a temperature zone from −100° C. to −200° C., and continuous oscillation of the semiconductor laser in a room temperature is difficult.
In view of the above, there is widely used a wavelength conversion technique, wherein light to be emitted from a semiconductor laser, a fiber laser excited by a semiconductor laser, or a like device is used as excitation light, and the excitation light is subjected to wavelength conversion by a wavelength converting element made of a non-linear optical crystal to generate light of a wavelength in a visible region or a mid infrared region. Propagation of excitation light in the wavelength converting element converts the energy of the excitation light into the energy of wavelength converted light. Extending the interaction length between the excitation light and the wavelength converting element increases the conversion amount into the wavelength converted light.
However, since the wavelength of excitation light and the wavelength of wavelength converted light are different from each other in wavelength conversion using a non-linear optical crystal, the refractive indexes of the excitation light and the wavelength converted light are greatly different from each other. For instance, in the case where lithium niobate (LiNbO3) as a representative non-linear optical crystal material is used as a material for a wavelength converting element, and green laser light (wavelength converted light of 532 nm wavelength) to be used in a display light source is obtained by incidence of excitation light of 1064 nm wavelength, the refractive indexes of the excitation light and the wavelength converted light are respectively 2.15600 and 2.23389 at e.g. 40° C., respectively. The phases of the excitation light and the wavelength converted light are gradually displaced from each other in the wavelength converting element, as the difference in refractive index between the excitation light and the wavelength converted light is increased. As a result, wavelength conversion cannot be performed efficiently.
In view of the above, conventionally, there are proposed, as methods for correcting a difference in refractive index between excitation light and wavelength converted light, quasi phase matching, wherein a non-linear optical crystal having a cyclic polarization inversion structure is used as a wavelength converting element, and birefringent phase matching, wherein wavelength converted light having the polarization which is different from excitation light, is generated. Use of these methods enables to match the phases between excitation light and wavelength converted light, or make the phases constant in an analogous manner at a certain temperature.
The refractive indexes 2.15600 and 2.23389 of excitation light and wavelength converted light at 40° C. are changed to 2.15642 and 2.23447 at e.g. 50° C., respectively. Accordingly, in any case, stable wavelength conversion cannot be performed, unless the temperature of the wavelength converting element is stabilized. In other words, it is necessary to perform wavelength conversion, while optimally adjusting a temperature of the wavelength converting element or an element temperature. Hereinafter, the optimal element temperature where the wavelength conversion efficiency is maximized is called as the phase matching temperature.
There are proposed, as conventional methods for adjusting the temperature of a wavelength converting element, a method (temperature constant control) of controlling the temperature of a wavelength converting element to a constant value, and a method (output constant control) of feeding back an output value to an input electric power to an excitation light source. Even if the temperature constant control is used, the element temperature is gradually changed from the phase matching temperature for the following reasons.
(1) The phase matching temperature is changed resulting from a variation in temperature of a semiconductor laser or a fiber laser as an excitation light source, or a variation in wavelength of excitation light to be outputted.
(2) The phase matching temperature is changed resulting from a change in refractive index due to an influence of a stress change accompanied by fixation of a wavelength converting element.
(3) The characteristic of a temperature sensor for measuring the temperature of a wavelength converting element is changed, and the element temperature to be monitored by the temperature sensor is changed from an actual element temperature, which means that the element temperature is apparently changed.
As the difference between the element temperature and the phase matching temperature is increased, wavelength conversion efficiency is lowered. In view of this, output constant control of keeping the output of wavelength converted light constant by increasing the output of excitation light is performed. In this control, however, an increase in electric power consumption cannot be avoided. Also, since the output of excitation light has a limit, the allowable temperature difference is limited. Further, in the case where a semiconductor laser is used to emit excitation light, the life of the light source is shortened, as the electric power to be supplied to the light source is increased. Furthermore, since the beam quality of wavelength converted light to be outputted in a condition that the element temperature and the phase matching temperature are different is poor, it is difficult to apply the light source to a processing apparatus or a medical apparatus requiring a high-quality laser output. In view of this, patent document 1 proposes control of periodically eliminating a difference between the element temperature and the phase matching temperature, in addition to temperature constant control and output constant control.
However, in the conventional art recited in patent document 1, the output constant control is temporarily suspended, and control of making the element temperature close to an element temperature where the output is maximized by detecting a variation in output while increasing or decreasing the element temperature is performed to eliminate a difference between the element temperature and the phase matching temperature. In this control, the output may be varied during the control.
In the case where a wavelength converting element is disposed in a consumer device such as a backlight device of a projection display device or a liquid crystal display device, since the temperature in the casing is gradually increased, a difference between the element temperature and the phase matching temperature may frequently occur, and the output may be unstable each time the difference occurs. As a result, white balance of the display device may be imparted. In view of this, output stability of a laser light source is demanded particularly for a display device.
Patent document 1: JP 2007-233039A